1. Field of Invention
This invention relates generally to adhereing a propylene polymer layer to an enamel coated metal surface and more particularly to a method for effecting an increase in the bond strength of the heat seal between the surfaces.
2. The Prior Art
Easy opening containers are known to the art. These containers are generally formed of metal and are provided with at least one pour opening. The pour opening generally occupies only a portion of the end panel of the container. Heretofore, the pour opening has generally been formed by scoring to define a tear strip. A pull tab is attached to the tear strip, and upon the application of a force, the pull tab is operative to separate the tear strip from the panel along the score line.
Although easy opening containers have been readily accepted by the public, deficiencies still remain in this type of container. One of these deficiencies is that the removable tear strip which is torn form the end panel in the opening of the can has sharp edges, and when thrown on the ground or otherwise improperly disposed of, remains as a nuisance which presents a cutting hazard to the public.
It has been proposed e.g., U.S. Pat. No. 3,616,047, to replace the metal tear strip with a plastic layered closure member to eliminate the cutting hazard as the removed portion will not have sharp edges. The closure member fabricated entirely from a thermoplastic resin, such as polypropylene or a laminate of the resin and a metal foil such as aluminum or steel is heat sealed to the surface of an enamel coated end panel having at least one performed opening, the enameled surface of the panel having been coated with a heat activatable, bond promoting layer containing a carboxyl modified polypropylene resin.
It has been further proposed, e.g., in copending application, Ser. No. 616,530 filed Sept. 25, 1975, in the name of Ernesto H. Manuel, assigned to the same assignee, to incorporate the carboxyl modified polypropylene resin in the enamel coating so that the propylene polymer layer can be bonded directly to the enamel coated metal surface without the imposition of a separate adhesion promoting layer between the propylene polymer layer and the enamel coated metal layer as previously disclosed in U.S. Pat. No. 3,616,047.
One drawback to heat sealing propylene polymers directly to enamel coated metal surfaces having the carboxyl modified polypropylene incorporated therein is that the peel strengths of the seal are only marginally acceptable for easy open container closure manufacture.